1. Field of the Invention
The present invention relates generally to radio communication technologies, and more particularly, to a method and an apparatus for transmitting a Hybrid Automatic Repeat reQuest-Acknowledgement (HARQ-ACK) of a Physical Downlink Shared Channel (PDSCH) in a traffic adaptive Time Division Duplexing (TDD) system when a TDD uplink and downlink configuration of a cell dynamically changes with uplink and downlink traffic.
2. Description of the Related Art
Long Term Evolution (LTE) technology supports a Frequency Division Duplexing (FDD) mode and a TDD mode.
FIG. 1 illustrates a frame structure in an LTE TDD system.
In the LTE TDD system, the length of each radio frame is 10 ms, and each radio frame is divided into two 5 ms half frames. Each half frame includes 8 0.5 ms time slots and 3 1 ms special domains. The 3 special domains include a Downlink Pilot Time Slot (DwPTS), a Guard Partition (GP) and an Uplink Pilot Time Slot (UpPTS), and each subframe includes two continuous time slots.
The transmissions in the TDD system include transmissions from a base station to a User Equipment (UE), i.e., downlink transmissions, and transmissions from the UE to the base station, i.e., uplink transmissions. According to the frame structure illustrated in FIG. 1, the uplink transmission and the downlink transmission in each 10 ms period share 10 subframes, and each subframe is assigned to the uplink transmission or the downlink transmission. The subframe assigned to the uplink transmission is referred to as an uplink subframe, and the subframe assigned to the downlink transmission is referred to as a downlink subframe.
The TDD system supports 7 types of uplink and downlink configurations, as shown in Table 1 below. In Table 1, “D” indicates downlink subframes, “U” indicates uplink subframes, and “S” indicates special subframes including the above-described 3 special domains.
TABLE 1TDDUL/DLconfig-switchingurationpointsubframe indexindexperiod01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
In order to increase the transmission rate of users, a newer version LTE TDD system has been proposed, which has the same HARQ transmission timing as the older LTE TDD system.
Specifically, the HARQ-ACK of PDSCH may be transmitted in a Physical Uplink Shared Channel (PUSCH) and a Physical Uplink Control Channel (PUCCH). For a timing from the PDSCH to the PUCCH, for example, the UE transmits the HARQ-ACK in the PUCCH of uplink subframe n, the PUCCH indicates the PDSCH in downlink subframe n−k and the HARQ-ACK released by Semi-Persistent Scheduling (SPS), where k∈K. The value of K is defined in Table 2 below, and K is a set of M elements, represented as {k0, k1, . . . kM-1}. Further, K relates to the serial number of a subframe and TDD uplink and downlink, and is referred to as a downlink association set. An element k is referred to as a downlink association element.
Hereinafter, downlink subframes corresponding to the downlink association set are referred to as a bundling window, i.e., for all k elements in K, the downlink subframes are a set {n−k, k∈K} composed of n−k elements. In a PUCCH subframe, each PDSCH of each downlink subframe is assigned PUCCH resources for transmitting the HARQ-ACK.
TABLE 2TDD UL/DLsubframe nconfiguration index01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8, 7, 5, 4, 11, 6———————6——775——77—
With the increasing requirements for data transmission rates, a traffic adaptive TDD technology has been proposed in the newer LTE version. By dynamically adjusting the ratio between uplink subframes and downlink subframes, the current uplink and downlink configuration more accords with the ratio between the current uplink traffic and downlink traffic, thereby improving the uplink and downlink peak rate of users and system throughput.
In the traffic adaptive TDD system, a TDD uplink and downlink configuration followed by the HARQ-ACK timing from the PDSCH to the PUCCH may be different from an actual TDD uplink and downlink configuration. For example, high-layer signaling or physical layer signaling may indicate a reference TDD uplink and downlink configuration, and whatever the actual TDD uplink and downlink configuration is, the HARQ-ACK of the PDSCH is transmitted according to the HARQ-ACK timing corresponding to the reference TDD uplink and downlink configuration indicated by the high-layer signaling or the physical layer signaling. For example, if the indicated reference TDD uplink and downlink configuration is TDD uplink and downlink configuration 2 and the actual TDD uplink and downlink configuration is TDD uplink and downlink configuration 0, 1, or 6, because downlink subframes of the actual TDD uplink and downlink configuration are a subset of downlink subframes of the reference TDD uplink and downlink configuration, all downlink subframes of the actual TDD uplink and downlink configuration may obtain uplink subframes for transmitting the HARQ-ACK of the PDSCH.
In an actual system, newer LTE version UEs and older UEs coexist. For the HARQ-ACK timing from the PDSCH to the PUCCH, the older UEs and the newer UEs may follow different TDD uplink and downlink configurations. For different TDD uplink and downlink configurations, when the HARQ-ACK of the downlink data is transmitted on the same one uplink subframe, there are different bundling windows, as illustrated in FIG. 2.
Referring to FIG. 2, letters “D” and “S” in subframes indicate downlinks subframes, and letter “U” indicates downlink subframes.
The newer UE transmits the HARQ-ACK of the PDSCH according to the HARQ-ACK timing of TDD uplink and downlink configuration 2, where the TDD uplink and downlink configuration used by the newer UE is the reference TDD uplink and downlink configuration indicated by the high-layer signaling or the physical layer signaling. The older UE uses the HARQ-ACK timing of TDD uplink and downlink configuration 0, where the TDD uplink and downlink configuration used by the older UE is a TDD uplink and downlink configuration indicated in system information (e.g., a System Information Block 1 (SIB1)) by a TDD UE that does not support adaptive traffic. When the newer UE and the older UE transmit the HARQ-ACK of PDSCH on the same uplink subframe 2 according to respective HARQ-ACK timings, the newer UE transmits the HARQ-ACK of downlink subframes 4, 5, 6, and 8, and the older UE transmits the HARQ-ACK of downlink subframe 6.
To transmit the HARQ-ACK of downlink subframes 4, 5, 6, and 8 corresponding to the newer UE, the downlink subframe 6 corresponding to the older UE has PUCCH format 1a/1b resources on the uplink subframe 2, and the downlink subframes 4, 5, and 8 have no PUCCH format 1a/1b resources on the uplink subframe 2. The PUCCH format 1a/1b resources are obtained according to the smallest Control Channel Element (CCE) index for scheduling the PDCCH.
For the newer UE, if PUCCH resources are reserved for each downlink subframe according to conventional technologies, because the number of downlink subframes in the reference TDD uplink and downlink configuration indicated by the high-layer signaling or the physical layer signaling is larger than the number of downlink subframes actually configured, not all of the PUCCH resources are actually needed, thereby wasting the PUCCH resources.
Accordingly, when the HARQ-ACK of the PDSCH in the traffic adaptive TDD system is transmitted, there is a problem how the newer UE is compatible with the older UE and a problem that the PUCCH resources are wasted in the prior art.